1. Field of The Invention
The invention relates to electrically driven optical switches employing the interferometric principle and more particularly to a low loss, electrically driven optical switch for circularly polarized light utilizing Faraday effect magneto-optic phase shifting.
2. Description of the Prior Art
Optical switches have been proposed using a variety of integrated optical waveguide structures. The balanced bridge, Mach-Zehnder modulator is one. In the known Mach-Zehnder modulator, which has two input ports and two output ports, an input light wave of usually plane polarized light is split into two equal components in an input 3-dB coupler. The two components then propagate over two arms of an interferometer and are subjected to independent phase shifts before being combined in an output 3-dB coupler. If no relative phase shift is present, all of the light "crosses over" to the diagonal output port. If a relative phase shift of .pi. radians is introduced between the two arms, the light goes "straight through" the input waveguides to the direct output port. If both relative phase states are achievable, the modulator may be used as a switch.
In the Mach-Zehnder interferometer just described, the relative phase shift between the two arms of the interferometer is brought about by means of an electrical field acting on an optical path in a material whose optical indices of refraction are altered by the electric field. The electric field is then used to adjust the phase delay in one path relative to the other.
In the conventional Mach-Zehnder design, the light is supplied to the interferometer in fibers from which the light is coupled to light conducting waveguide paths diffused into a lithium niobate crystal (LiNbO.sub.3, or more popularly "LNO") which has the necessary electro optical properties. Such a configuration is discussed in an article entitled "Waveguide Electro-optic Modulators" by Rod C. Alferness appearing in the IEEE Transaction on Microwave Theory and Technique, Vol. MTT30, August 1982, page 1129-1131.
The planar waveguide sections formed in a LNO switch of the Mach-Zehnder design have an optical mode size and shape differing from that in a fiber. The differences between a fiber and an integrated waveguide result in a substantial amount of fiber to waveguide coupling loss. Conventionally the losses are from one to two dB at the input connections and an equal amount at the output connections. When switches are connected in tandem, each tandem connection increases the optical loss. The optical loss in dB is doubled to give the equivalent electrical loss, producing an undesirably high total loss for each tandem connection. More than one tandem connections are therefore generally impractical. Removal of the coupling loss in an optical switch would accordingly be highly desirable.
The interferometric concept has also been used for many kinds of fiber sensors. These devices detect changes in pressure, strain, or temperature. Those that are sensitive to electric or magnetic fields generally have an intermediate field-to-strain transducer. The intermediate mechanical or thermal inputs for such sensors make for a slow response, which is undesirable for a switch. Thus for optical switching, where switching speed is important, a direct response to electro-magnetic field change is preferable. Since the amorphous silica fibers commonly available for light transmission have a fixed, average inversion symmetry, such that low frequency and dc electric fields have no effect, their use for electro-optical switching is ruled out.
The alternative to electro-optical sensing is magneto optical sensing, using Faraday effect phase shifting. This effect has been used to sense currents in high voltage power line without introducing electrical paths between the high voltage line and near-ground potentials used for the sensing equipment. In a known arrangement described in the Rashleigh et al article (discussed hereinafter), the current is sensed by detecting changes in linear polarization through a single-fiber loop around a conductor. The sensors herein described usually sense small, analog phase changes rather than the larger .+-.90.degree. phase changes required for optical switching.